1. Field of the Invention
The present invention generally relates to methods for manufacturing semiconductor devices. More specifically, the present invention relates to methods for enhancing the performance of semiconductor devices and the semiconductor devices obtained with the methods thereof. More particularly, the present invention also relates to methods for rounding corners and smoothing surfaces of semiconductor devices.
The present invention also relates to methods for manufacturing fin-based devices. More particularly, the present invention also relates to methods for rounding corners and smoothing surfaces of fin-based devices and the fin-based devices made thereof.
2. Description of the Related Technology
Scaling down of planar bulk CMOS devices has become a major challenge in the semiconductor industry. High channel doping, band-to-band tunneling across the junction and gate-induced drain leakage, short channel effects are some of the challenges that need to be overcome. Whereas at the beginning, device geometrical shrinking already gave a lot of improvements in IC performance, nowadays new techniques, methods, materials and device architectures have to be introduced beyond the 90 nm technology node. Multi-gate field effect transistor (MUGFET), also often referred to as fin-based semiconductor device or FINFET, is one of the promising candidates for further scaling down to 32 nm or less. Due to their three dimensional architecture, with the gate electrode wrapped around a thin semiconductor fin, an improved gate control (and thus less short channel effects) over the channel could be achieved by using multiple gates. Depending on the shape of the gate electrode, different types of MUGFETs can be defined. A double-gate finfet is a multi-gate device where the gate electrode only controls the conductivity of the two sidewall surfaces of the fin. Such a device is also often referred to as a double-gate device. An omega-gate finfet (Ω-gate finfet) is a multi-gate device where the gate controls the conductivity of the two sidewall surfaces and the top surface of the fin. An U-gate finfet is a multi-gate device where the gate controls the conductivity of the two sidewall surfaces and the bottom surface of the fin. A round-gate finfet is a multi-gate device where the gate controls the conductivity of the two sidewall surfaces, the top surface of the fin and the bottom surface of the fin. However with the introduction of MUGFETs, new problems arise.
One problem is related to the etching of the fin structures (typically done in the patterning process of the fins). Whereas for planar bulk CMOS devices current conducts on the top surface of the semiconductor wafer,—depending on the type of MUGFET—the current in MUGFET devices takes place not only on the top surface of the device, but also along the sidewalls of the fins. The etching of the fins induces damage of the sidewall surfaces of the fin, leading to reduced carrier mobility along these surfaces. For optimal device performance the sidewalls should be as smooth as possible. The surface roughness of the sidewalls of the fin should be as small as possible.
Another problem is the corner effect. In the corners of the fin, typically with an angle of 90 degrees, the electrical fields during device operation are different than in the planar region of the device. This leads to a different threshold voltage in the corner regions of the MUGFET compared to the threshold voltage along the sidewalls of the MUGFET, resulting in a degradation of the subthreshold characteristics. Therefore, for achieving good device performance, it is desirable to eliminate as much as possible the sharp corners in the fins.
One known approach to smooth the sidewalls and to round the corners of the fin is made through a combination oxide removal and hydrogen anneal of the fins, as explained in a paper by R. J. Zaman et al. “Effect of hydrogen annealing process conditions on nano scale silicon (011) fins,” Mater. Res. Soc. Symp. Proc. Vol. 872 p. 37-41 (2005). Due to the hydrogen annealing, a reflow of the atoms at the semiconductor surface of the fin will occur tending to minimize the surface energy, thus resulting in minimizing the surface area, thus resulting in rounded corners. A disadvantage of this method however is the narrow temperature/pressure window. For example, at atmospheric pressure no reflow occurs below 875 degrees Celsius while above 925 degrees Celsius silicon evaporation occurs. Three other disadvantages of this method are: the undercut at the bottom fin which will decrease the stability of narrow fins; the control of the radius of the corner rounding which is crucial for controlling the profile of narrow fins and, as a consequence from the latter disadvantage, the limitation on fin width since small fin width variations will result in discontinuous fins because of Si migration.
As such, there is a need to overcome these disadvantages mentioned in the prior art.